The present invention relates to a hydraulic booster for a fluid brake.
Hitherto, hydraulic boosters for fluid brakes are known through, for instance, U.S. Pat. Nos. 3,747,473 and 3,815,364.
The hydraulic booster disclosed in U.S. Pat. No. 3,747,473 is illustrated in FIG. 3. In the normal state shown in FIG. 3, this apparatus is arranged such that a piston of a master cylinder (not shown) is coupled to a power piston 10 by means of a coupling member 85, a ball valve 83 abuts against a valve seat 84 to close a high-pressure channel 11 connected to a high-pressure fluid source (not shown) through a high-pressure port 86. In addition, an inner member 81 is separated from the ball valve 83, and a channel 16 connects a pressure chamber 45 to a low-pressure port 80, so that the pressure chamber 45 is in an unpressurized state.
Subsequently, if a push-rod 37 is pushed, a tip of the inner member 81 is brought into contact with the ball valve 83, thereby closing the channel 16 connecting the pressure chamber 45 to the low-pressure port 80. If the inner member 81 is further pushed, the ball valve 83 is separated from the valve seat 84, which in turn causes the high-pressure channel 11 to open, thereby allowing the pressure chamber 45 to be communicated with the high-pressure fluid source.
The high-pressure fluid which has flowed into the pressure chamber 45 then pushes the power piston 10 leftwardly, as viewed in the drawing, which in turn causes the piston of the master cylinder (not shown) to advance via the coupling member 85, thereby feeding the fluid contained in the master cylinder into a wheel cylinder.
Simultaneously, since the high-pressure fluid in the pressure chamber 45 pushes an outer member 82 rightwardly, as viewed in the drawing, the push-rod 37 transmits a pressure proportional to the hydraulic pressure within the pressure chamber 45 to the operator.
The hydraulic booster disclosed in U.S. Pat. No. 3,815,364 is arranged such that, in the same way as the apparatus shown in FIG. 3, a high-pressure fluid from a high-pressure fluid source is introduced into a pressure chamber to press a power piston so as to press a piston in a master cylinder. Specifically, a high-pressure channel for communicating the pressure chamber and the high-pressure fluid source and a channel for communicating the pressure chamber and a low-pressure port are opened and closed by a spool valve, so that this apparatus is capable of transmitting a pressure proportional to the hydraulic pressure in the pressure chamber to the operator.
The conventional apparatus shown in FIG. 3 and disclosed in U.S. Pat. No. 3,747,473 is compact as compared with a booster of a type which makes use of the pressure differential of the air, and can be conveniently used since a reactionary force proportional to the pressing force of the piston can be transmitted in the same way as the booster which makes use of the pressure differential of the air.
However, in the conventional apparatus shown in FIG. 3, the valve seat 84 for opening and closing the high-pressure channel 11 is pushed directly in the sliding direction of the inner member 81, and the sliding speed of the inner member 81 and the moving speed of the ball valve are set to be equal. Consequently, the internal pressure of the pressure chamber 45 rises suddenly.
In addition, since the internal pressure of the pressure chamber 45 rises suddenly, the reactionary force which is transmitted to the push-rod 37 also increases sharply. Consequently, the advancing speed of the push-rod 37 declines, and the rise in the hydraulic pressure in the pressure chamber 45 becomes dull. Hence, the hydraulic pressure in the pressure chamber 45 is subjected to a change in the form of broken lines, and the hydraulic pressure continues to be affected by that change, so that the hydraulic pressure changes in steps, thereby making it impossible to perform a smooth braking operation.
In addition, since the hydraulic booster disclosed in U.S. Pat. No. 3,815,364 employs a spool valve, precision machining of fitting portions of said spool valve is required, and there are cases where leakage of the liquid occurs if a gap of said fitting portions changes due to a change in the temperature of the liquid.